1. Field of the Invention
This invention relates to a method for producing flame retardant wood, and more particularly, to a method for producing modified wood which is rendered flame retardant for a very long time by forming a metal oxide in the wood and preventing the metal oxide from being leached out in water.
2. Prior Art
Wood has characteristic features of "burning," "decay" and "dimensional instability." Utilization of wood is often restrained by these features when they are regarded drawbacks in a certain application. For example, according to the Japanese building regulation amended June, 1990, wood may be used in open areas insofar as it satisfies a fire penetration resistance/flame shielding time of 60 minutes and 20 minutes for Types A and B, respectively. Wood as such does not clear this standard.
Wood engineers strove to overcome these drawbacks of wood. We found that wood can be modified so as to exhibit satisfactory flame retardance, decay resistance, and dimensional stability by impregnating wood with a silicon alkoxide and forming and fixing silicon oxide in wood cell voids, preferably in voids of wood cell walls, through hydrolysis and polycondensation (see Journal of Japan Wood Research Society, 38 (11), 1043, 1992). This technique is based on a sol-gel method using metal alkoxides. A starting solution of metal alkoxide, water, alcohol and catalyst converts into a sol of metal oxide through hydrolysis and self-polycondensation of the metal alkoxide. As reaction proceeds further, the solution becomes a gel. If this reaction takes place in wood cells, metal oxides formed therein convert wood into wood-inorganic composites.
The recent research revealed that the process of forming wood-metal oxide composites largely depends on the rate of hydrolysis of the metal alkoxide used and subsequent self-polycondensation and that the distribution of metal oxide within wood cells largely varies with preparation conditions. Consideration is now made of the conversion of wood into inorganic composites using a silicon alkoxide having a low rate of hydrolysis. When a moisture-conditioned wood piece (all water in wood exists solely in cell walls as bound water) is used, hydrolysis and poly-condensation of silicon alkoxide takes place only within cell walls where bound water exists, resulting in a wood-inorganic composite wherein cell cavities are void. This wood composite maintains the characteristics of wood which are lightweight, strong, and heat insulating. The modified wood is given dimensional stability, flame retardance and decay resistance while maintaining the porous feature of wood (see Journal of Japan Wood Research Society, 39 (3), 301, 1993). When a water-saturated wood piece (wood is full of water not only within cell walls, but also in cell cavities) is used, however, obtained is a wood-inorganic composite wherein metal oxide fills not only within cell walls, but also in cavities (see Journal of Japan Wood Research Society, 39 (3), 301, 1993).
However, if the metal alkoxide used is changed, obtained is a wood-inorganic composite having a completely different distribution of metal oxide. When a titanium alkoxide having a high rate of hydrolysis is used for a moisture-conditioned wood piece, titanium oxide is formed only within cell cavities. In the case of a water-saturated wood piece, titanium oxide is formed only on the outer surface of wood and no composite with metal oxide is formed within the wood piece (see Journal of Japan Wood Research Society, 39 (3), 308, 1993).
The relationship of the distribution of metal oxide in cells to the imparted function is examined on the basis of these findings. It was found that if a metal oxide composite is selectively formed within cell walls, various functions are effectively exerted by forming only small amounts of metal oxides. Such wood composites have incorporated therein inorganic substances, typically metal oxides resulting from silicon alkoxides, boron alkoxides and phosphorus alkoxides. When treatment is done at temperatures at which wood is not pyrolyzed, the latter two alkoxides form unstable metal oxides which are easily leached out in water.
Additionally, the above-mentioned metal alkoxides must be carefully handled on actual use since they have a high vapor pressure and toxicity.